We propose to study the interaction of metals with proteins involved in regulated heavy metal resistance in bacteria. We have chosen two well-studied examples: the gene products of the mer operon from Gram negative transposon Tn21 that confers mercury resistance; and the gene products of the ars operon from E. coli plasmid R773 that confers arsenical and antimonial resistance. In the mer operon, Hg(II), Cd(II), and Zn(II) interactions with the MerR metalloregulatory protein and mutants that have altered metal activation specificities will be compared at the level of active-site structure. In the ars operon, As(III) and Sb(III) binding sites on ArsR and ArsD, two co-regulatory proteins that control ars gene expression by an arsenite/antimonite-dependent mechanism, and ArsA, the ATP-dependent and M(III)-activated arsenite/antimonite ion transporter, will be defined. Site-directed mutagenesis will be used to determine the residues involved in As(III) and Sb(III) binding sites.